1. Field of the Invention
The present invention concerns a method and apparatus for countertyping a tint by formulating a composition made from a collection of basic tints, without measuring the reflectance curve of the tint to be countertyped. In particular, the invention is a method and apparatus permitting an absolute formulation of tint parameters, rather than a relative formulation, thereby enabling tint formulation using a tristimulus colorimeter and not requiring a spectraphotometer as previously needed for absolute tint formulation.
2. Prior Art
Numerous industries are obliged to paint their production with paints specifically adapted to the types of their manufactures in tints sometimes very numerous. There are two results:
(1) mass produced paints available in quantity from large manufacturers are not suitable; and, PA1 (2) the amounts per tint (often less than 100 kg) are not profitable and result, besides a high cost, in difficulties of supply not only from the quality standpoint but also from the delivery standpoint. Few paint manufacturers are structured for responding to this demand which nevertheless is increasing considerably. PA1 A=first basic tint, PA1 B=second basic tint, PA1 W=basic white, PA1 C=concentration of the basic tint concerned, PA1 R is the internal reflectance within the object; PA1 Ke is the external correction coefficient, subject to the FRESNEL law; ##EQU4## Ki is the internal correction coefficient (a value close to 0.65). PA1 .DELTA.C=difference of concentration PA1 .DELTA.T=difference of a trichromatic component PA1 the spectrophotometric method permitting an absolute formulation; or PA1 the colorimetric method permitting only a differential formulation. PA1 reproducibility of the filters; PA1 reproducibility of the illuminant; PA1 reproducibility of the photo-receiving cells; PA1 background noise of the particular electronic system and its environment; PA1 the structure and alignment of the optical geometry of the apparatus for collection of the reflected energy; and, PA1 the nature and optical geometry of the apparatus for emission of luminous energy.
The known manner of dealing with this problem at the level of the paint manufacturers requires a relatively high investment and qualified staff.
In fact, up to now, a spectrophotometer has been used for custom tint formulation which determines the reflectance spectrum, or spectral reemission curve, of the sample to be countertyped, i.e. the relationship between R, the reflectance coefficient, and the absorption K and diffusion S coefficients of the tint of the sample, over the whole visible spectrum (400-700 nm) taken at regular wave length intervals, in accordance with the Kubelka-Munk law ##EQU1## for each given wave length .lambda..
Knowing the reflectance spectrum of the basic tints and applying the relationship: ##EQU2## where the meaning of the symbols used is as follows: M=mixture of the basic tints,
the formulation of the tint recipes may be calculated in this manner. This calculation is obviously very complex and requires the use of a computer.
The known manner of countertyping basically relied on relative comparison of a needed tint with a proposed sample, to obtain information regarding additions of further tints to cause the proposed sample to more closely approximate the needed tint.
The state of the art, at least as of February 1985 thus permitted corrections of existing formulae to be obtained (i.e., relative formulation of tint parameters) from the differences measured by a filter colorimeter, also known as a "tristimulus colorimeter", between the values measured on the standard to be countertyped (the needed tint) and the values measured on the tint obtained from a formula (the proposed tint), the parameters of which are known and preferably are, at the outset, close to the tint of the standard.
This type of process is described, for example, in U.S. Pat. No. 3,690,771-Armstrong et.al.; in U.S. Pat. No. 3,601,589-McCarthy; and in the publication "Batch Color Correction by Tristimulus Colorimeter" of Modern Paints and Coatings, September 1980, pages 41 to 44. This process is shown in FIG. 2, a block diagram, and can be explained as follows with reference to Steps A through D: